The present invention refers to semiconductor lasers and methods of making same.
Exemplary of a prior art semiconductor laser is the arrangement shown in FIG. 1, which is currently referred to as a SIBH (Semi-Insulating Buried Heterojunction) structure.
Specifically, in FIG. 1 reference numeral 1 denotes a n-type substrate defining a mesa like structure laterally confined by an Fexe2x80x94InP semi-insulating layer 2. Reference numeral 3 denotes the MQW active (i.e. lasing) layers over which a further p layer 4 and a SiO2 mask 5 are provided. Finally, reference numeral 6 indicates an n-InP layer superposed to the Fexe2x80x94InP semi-insulating layers and adjoining the sides of mask 5 as an anti-diffusion layer to prevent Znxe2x80x94Fe interdiffusion.
The captioned structure is conventional in the art and may be resorted to for manufacturing, i.a. SIBH-DFB (Distributed Feed Back) lasers operating e.g. in the 1.3 micrometer wavelength range.
A problem currently encountered with such structures is that, while being fully effective at room temperature (or lower), at higher temperatures (around 80xc2x0 C. or above) the semi-insulating behavior of the Fexe2x80x94InP layers is no longer satisfactory. This is essentially due to a fairly large leakage current being established across the Fexe2x80x94InP regions.
Furthermore, since the Fexe2x80x94InP layers are in contact with the MQW active layers, the Fe atoms will diffuse into the MQW layers and the p-InP layer (denoted 4 in FIG. 1) located above the MQW layers. This results both in reduced reliability and in high series resistance. Specifically, a 0.2-0.3 micrometer diffusion region of Fe into the MQW region can be observed by low temperature cathode luminescence. Also, a 0.2-0.3 equivalent reduction of the p-InP width (layer 4) caused by Fexe2x80x94Zn interdiffusion can be detected by series resistance measurements.
In order to reduce the leakage current, an alternative SIBH structure has been proposed where a wide-bandgap layer is arranged between the original mesa arrangement and the semi-insulating layer: see S. Asada et al., xe2x80x9cAnalysis of Leakage Current in Buried Heterostructure Lasers with Semiinsulating Blocking Layersxe2x80x9d, IEEE Journal of Quantum Electronics, Vol. 25, No. 6, June 1989, pp. 1362-1368.
The leakage current can thus be greatly reduced due to a large turn-on voltage of the wide-bandgap material (e.g. a thin InGaP layer).
The main disadvantage of that arrangement is related in the growth of the wide-bandgap layer by means of MOCVD (Metal Organic Chemical Vapor Deposition). The high mismatch between InP and InGaP does not in fact permit an InGaP layer to be grown even in the range of 100 nanometers, which causes reliability deterioration under high temperature working conditions.
The object of the present invention is thus to provide a further improved solution overcoming the disadvantages outlined in the foregoing.
According to the present invention, that object is achieved by means of a semiconductor laser structure having the features called for in the annexed claims. The invention also relates to the corresponding manufacturing method.